Recently, an ADSL (Asymmetric Digital Subscriber Line) communication system, an HDSL (High-rate Digital Subscriber Line) communication system, an xDSL communication device system such as an SDSL, which perform high-speed digital transmission of several megabits/second using an existing copper cable for telephone, are attracting attention as cable digital communication systems. The xDSL communication system used in these is known as a DMT (Discrete MultiTone) variable demodulation system. This system is standardized in T1.413 of ANSI and elsewhere.
In this digital communications system, when an xDSL transmission path and an ISDN transmission path of an ISDN communications system, which is a half-duplex communications system, are adjacent to each another when for instance they are converged together or the like in a midway collection guide, an xDSL transmission along the xDSL transmission path is affected by interference noise from other circuits such as the ISDN transmission path, leading to problems such as a loss of speed. Various countermeasures have been adopted.
FIG. 10 shows interference noise from an ISDN transmission path 2 from a central office (CO) 1 affecting an ADSL transmission path 3 which is an xDSL transmission path, when the ISDN transmission path 2 and the ADSL transmission path are converged together or the like midway at a collection guide.
Here, when seen from the ADSL remote terminal end transceiver unit (ATU-R) 4, which is a communication device at the terminal side of the ADSL communication system side, the interference noise transmitted through the ADSL transmission path 3 by the office side device (ISDN LT) 7 on the ISDN transmission system side is called FEXT (Far-End Cross Talk), and the interference noise transmitted through the ADSL transmission path 3 by the terminal device (ISDN NT1) 6 on the ISDN transmission system side is called NEXT (Near-end cross talk). In particular, a midway collection guide or the like causes coupling between the ADSL transmission path 3 and the ISDN transmission path 2, which becomes adjacent to the ADSL transmission path 3, whereby the noise is transmitted through the ADSL transmission path 3 to the ADSL terminal side device (ATU-R) 4.
When seen from the ADSL central office end transceiver unit (ATU-C) 5, which is an office side device on the ADSL communication system side, contrary to the case seen from the ADSL remote terminal end transceiver unit (ATU-R) 4, the interference noise transmitted by the central office end device (ISDN LT) 7 on the ISDN transmission system side is a NEXT noise, and the interference noise transmitted by the terminal device (ISD NNT1) 6 on the ISDN transmission system side is a FEXT noise.
Here, in overseas ISDN communication systems, all upstream and downstream transmissions are duplex transmissions and carried out simultaneously. Therefore, when seen from the ADSL remote terminal end transceiver unit (ATU-R) 4, the NEXT noise generated from the terminal device (ISDN NT1) 6 on the ISDN transmission system side which is nearer to the ADSL remote terminal end transceiver unit (ATU-R) 4, is dominant, is dominant, that is, its effects are considerable.
Consequently, during the training time of an ADSL modem (not shown in the diagram) provided to the ADSL remote terminal end transceiver unit (ATU-R) 4, training is carried out while both the FEXT noise and the NEXT noise are being generated at the same time, and the characteristics of the NEXT noise components with a large influence is measured. A bit map is then made to determine the gain and number of transmission bits in each channel suitable for characteristics of the noise. In addition, to improve the transmission characteristics, coefficients for a Time domain Equalizer (TEQ), to perform time domain equalization processing, and a Frequency domain Equalizer (FEQ), to perform frequency domain equalization processing, are for instance converged and determined, and one coefficient table is provided for the TEQ and the FEQ respectively.
As a result such problems are not caused for overseas digital communications devices as mentioned above. However, TCM-ISDN systems performing half-duplex communication known as Ping-Pong systems, wherein upstream and downstream data transmissions are switched by time division, are used as existing ISDN communication systems in countries such as Japan. Consequently, when the half-duplex transmission path is adjacent to another transmission path in a collection guide or the like, the NEXT noise and the FEXT noise from the half-duplex transmission path affect the communication terminal connected to the other transmission path adjacent to the half-duplex transmission path.
For this reason, when a half-duplex communication system such as a Japanese TCM-ISDN system is used, an ADSL terminal side device (ATU-R) capable of handling entirely half-duplex ISDN communication systems of overseas countries and the like installs only one coefficient table trained under the situation where both the FEXT noise and the NEXT noise are being generated at the same time, and upstream and downstream communications on the ISDN transmission path are time-divided by a TCM system. Therefore, even when the noise components affecting the terminal connected to the transmission path adjacent to the ISDN transmission path are switched to the NEXT noise and the FEXT noise, since these are gathered together in a single coefficient table it tries to settle to one coefficient table. Whenever the amount of the noise and the change in the character took place, the amount of error at the terminal worsens, the speed at which the amount of error can be improved is slowed, and other problems arise.
In the light of the problems described above, it is an object of the present invention to provide a digital communication system which can improve the influence by the noise, even when interference noise is received from a half-duplex transmission path for TCM-ISDN communication and the like via a digital transmission path adjacent thereto, and extend communication processing in each transmission rate, and, in addition, improve the transmission rate in the service area.